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"If there had been a police escort maybe what happened yesterday could have not have happened," Biti said.

"(A) police escort would have warned oncoming vehicles of a VIP arriving. I think authorities must understand the omission.

"We hope that this omission will be rectified, that the prime minister must be given the protection that ought to be accorded to a prime minister."

Biti said the MDC would launch its own investigation.

Analysts say the crash raises suspicions of foul play. One former U.S. diplomat called for an outside investigation, saying it was not the first time one of Mugabe's political foes had been killed or injured in a car crash.

Tsvangirai, Zimbabwe's main opposition leader, took office last month under a power-sharing deal with Mugabe following a contentious election.

The MDC reached the agreement with Mugabe in September after months of angry dispute that included violence. More than 200 deaths, mainly opposition supporters, were reported leading up to and after the election.

"I'm skeptical about any motor vehicle accident in Zimbabwe involving an opposition figure," said Tom McDonald, the U.S. ambassador to Zimbabwe from 1997 to 2001. "President Mugabe has a history of strange car accidents when someone lo and behold dies -- it's sort of his M.O., of how they get rid of people they don't like." Watch more on the fatal crash »

McDonald cited the car crash deaths of Defense Minister Moven Mahachi in 2001, Employment Minister Border Gezi in 1999 and Elliot Manyika, a government minister and former regional governor, last year.

"So when I hear that Tsvangirai was in an accident, it gives me pause," McDonald said.

Now an attorney with the Washington law firm Baker Hostetler, he urged a full independent investigation.

However, he added that traffic accidents are common in Zimbabwe. The highway Tsvangirai was traveling on was only two lanes and tractor-trailers were common, McDonald said. Vehicles in the country were often in bad shape and drivers inexperienced.

"It's certainly plausible that this was just one of those tragic things," he said.